Photoelectric apparatus and method of operating the same



Oct. 6, 1936. J. H DE BOER ET AL 2,056,392

PHOTOELECTRIC APPARATUS AND METHOD 0F OPERATING THFI SAME Filed Nov. 19,l19;2

Patented Oct. 6, A1.936

UNITED STATES PATENT OFFICE PHOTOELECTRIC APPARATUS AND METH- 0D 0FOPERATING THE SAME War!! Application November 19, 1932, SerialNo.,643,320 In Great Britain June 14, 1931 We have already devised aphoto-electric tube comprising a photo-electric electrode in which areincorporated particles of a chemical 'compound mixed with particles of aphoto-electric metal. Such a photo-electric tube is described andclaimed in our copending application, Ser. No. 452,785 nled May 15,1930. In fact, this tube comprises a cathode containing a layerconsisting of a photo-electric material, for instance, caesium, which iscarried by another layer containing particles of a chemical compound,for instance, calcium iluoride, mixed with particles of a photo-electricmaterial, for instance, caesium. This layer may be applied in its turnto a metal substratum by whichythe supply of the electron current isfacilitated.

In our copending application, Ser. No. 577,740, iiled Nov. 28, 1931, isdescribed and claimed a photo-electric tube having a cathode providedwith a layer containing particles of a photo-electric metal, forinstance caesium mixed with particles of an oxide of such a metal, forinstance, caesium oxide, and particles of another metal, for instance,silver particles.

Experiments have proved that the sensitiveness of such extremelysensitive electrodes, i. e. the number of electrons emitted withdefinite exposure, is not constant, but decreases on irradiating thephoto-electric electrode, provided a tension is applied to theelectrodes. larger as the intensity of the exposure is greater. It hasbeen found, for instance, that a tube which has been inoperative forsome time and whose sensitiveness amounted to 30 micro-amperes perlumen, proved to have a smaller sensitiveness on irradiating theelectrode with 0,5 lumen (2680 KJ. After one hour, for instance, thesensitiveness amounted to 20 micro-amperes per lumen.

When being inoperative for some time the sensitiveness of such a. tubeincreases again and after a suiilcient rest it attains again itsoriginal value. This time interval may be considerably reduced by ashort heating of the tube, for instance, for 1 minute to a temperatureof C.

This variation of the sensitiveness of the photo-electric tube isfrequently a considerable nuisance. The present invention has for itspurpose to obviate this drawback and relates to a photoelectric tubehaving a photo-electric electrode in which is incorporated a mixture ofparticles oi.' a chemical compound and particles of a photo-electricmaterial, and in addition to a method of operating such a tube. y

According to .the invention the photo-electric electrode of a tube ofthe type referred to above This decrease will bev isv irradiated withlight having such a composition that the ratio between the part of thelight having a wave length of more than 5800 vand the light source(electric vacuum or gaslled incandescent lamp), in which light isemitted by an incandescent body. In fact, it has been found, that thelight rays having a wave length of less than 5800 i. e. essentially the.green, blue and violet rays cause the decrease of the sensitiveness,whereas on the contrary rays having a wave length of more than 5800 moreparticularly the red and the adjacent infrared rays further a cohstantsensitiveness. Consequently, if care be taken thatthe light with whichthe photo-electric electrode is irradiated, is relatively poorer in rayshaving a wave length of less than 5800 as compared with normal whitelight, then the decrease of sensitiveness is smaller during operation ofthe tube.

The invention is particularly advantageous when using a photo-electrictube having a photoelectric electrode containing a mixture of particlesof a photo-electric metal, particles of an oxide of such a metal and inaddition'particles of another metal.

It has been found that, if the light used for the irradiation isrelatively rich in rays having a Wave length above 5800 the ratiobetween this part of the light and the part having a wave length under5800 is at least 10% larger than -this ratio for white light emitted by.an incandescent bcdy, a considerable improvement with respect to aconstant sensitiveness is yielded. Still better results are obtained ifthe difference between the said ratio amounts to 20% or even to 30%. Itis also possibleto proceed further and to irradiate the photo-electrictube with light substantially containing rays only having a wave lengthof more than 5800 y i The invention may be used in various ways. It ispossible, for instance, to use a light source emitting substantially noor only a small quantity of rays having a wave length under 5800 It isadvantageous to use for this purpose an electric discharge tube filledwith neon or sodium vapour.

In many cases it will be advisable, however, to provide a filter infront of the aperture of the photo-electric electrode through whichenter the rays, said filter absorbing the light having a wave length ofless than 5800 better than light having a larger wave length. In thiscase ordinary white light sources may be used, for instance, electricincandescent lamps or the daylight. Preferably the filter is of such anature that it is substantially transparent only for light having a wavelength above 5800 Excellent results have been obtained with filtersshowing a red colour when being traversed by light rays.

of the wall of the tube. It is possible, for instance, to make the wholewall of the tube or only the window for the passage of the rays oi.' thematerial of which the lter consists. In general it will be simplest toapply the lter to the part of the tube wall transparent to the raysiniuencing the photo-electric electrode.

It is also possible to assemble the ilter andthe light source, forinstance, by using an electric incandescent lamp having a red bulb as isusually employed in photography for instance.

The invention will be more clearly understood by reference to theaccompanying drawing, representing, by way of example, a form ofconstruction thereof.

Fig. 1 shox s a photo-electric tube according to the invention, and

Fig. 2. shows a detail of this tube.

The photo-electric tube shown in Fig. 1 is highly evacuated and has aspherical wall I to which is sealed a tube 2. 'I'his tube is providedwith a foot 3 carrying the electrodev 4. As clearly appearsy from Fig. 2this electrode is constituted by an almost entirely `closed metal ring,whose ends are connectedI to the supply conductors 5 and 6.

On the wall I is provided a metal layer 1 consisting, for instance, ofsilver, said layer being electrically connected to the supply conductor8 and coated with a layer 9 consisting of thoroughly mixed metal(silver) particles, particles of a photo-electric metal (caesium) andparticles of an oxide of a photo-electric metal (caesium oxide) On thisintermediate layer is provided a thin caesium layer I0.

This device is established in the following way. First of all a layer ofa metal oxide,` for instance, a silver oxide layer is produced which maybe done by oxidizing a metal layer produced, for instance, byvaporization in a vacuum. However, the metal oxide layer may also beobtained in another manner, for instance, by Vaporizing a metal oxideprovided on an ncandescible member mounted within the tube. Afterestablishing the metal oxide layer an excess ofl a photo-electric metal,for instance, an alkali metal, is introduced into the tube, if desired,after a further exhaustion of the tube, whereupon the tube, which is notconnected to a vacuum pump, is heated to such a temperature that themetal oxide is reduced and an oxide of the photo-electric metal isproduced. The excess of photoelectric metal is made to penetrate intothe produced mixture of metal particles and oxide particles. It isadvantageous to heat the tube gently for this purpose, for instance, toabout 100 C. On the layer produced is also formed a thin alkali' metallayer.

On reducing the metal oxide care must be taken that the tube is notheated to such an extent that the oxide of the photo-electric metaldecomposes or volatilizes, or that the produced The filter mayadvantageously constitute part A* metal particles C103'- 'Ihe method ofestablishing the photo-electric electrode is more amply set out in ourUnited States patent application Serial No. 577,740.

It has been found that the passage of current produced in this tube whenapplying a tension to the anode by irradiating the photo-electricelectrode with white light, gradually-decreases with a constantintensity of irradiation. This drawback is avoided by irradiating thephoto-electric electrode with light containing no or only a smallquantity of rays having a wave length of less than 5800 For this purposethe part II of the wall of the tube which part is not coated with thephoto-electric material and serves for the passage of the light rays, isexternally coated with a filter I2 (Fig. l) showing a red colour onbeing traversed by light and consisting, for instance, of a lacquerlayer containing a red colouring agent as is used, for example, in themanufacture of red lamps serving for photographic purposes.

It will be appreciated that the filter need not always be applied to theouter side of the window. It is also possible that the wall of the tubeor the window is entirely constituted by the lter, for instance, byusing a suitable red glass. Furthermore, it is also possible"to arrangethe filterso as to be loose from the wall of the tube and to locate itwithin the tube or in front of the window in the way of the light rays.

The rays having a wave length of less than 5800 are substantiallyentirely absorbed by the red illter, so that only rays having a greaterwave i length, viz. the red rays, may reach the photoelectric electrode.It has been found, that the sensitiveness of this electrode ispractically constant on being irradiated with red rays. In a tube whosesensitiveness has decreased by irradiation with white light, the timerequired for reestablishing the original sensitiveness may be reduced byirradiating the photo-electric electrode with red light.

What we claim is:

The method of operating a photo-electric tube comprising the step ofirradiating said tube with light, comprising a greater portion of lightof Wave length of more than 5800 'units than of that less than 5800units, the difference between the two portions of the light being atleast ten percent of the sum of their amounts, whereby the sensitivityof the said tube remains constant irrespective of the time ofirradiation.

JAN HENDRIK DE BOER. MARTEN CORNELIS TEVES.

